JP2581522C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of an X-ray tomography apparatus (hereinafter, referred to as an X-ray CT apparatus). 2. Description of the Related Art Conventionally, an X-ray CT apparatus obtains an X-ray tomographic image (hereinafter, referred to as a tomographic image) for a desired slice plane of a subject, for example, a patient while keeping the position of the patient fixed. By exposing X-rays from the X-ray tube while rotating the X-ray tube around the patient in a vertical plane having the slice plane, all projection data from all directions on the slice plane is collected. Image reconstruction is performed based on all projection data, and a tomographic image of a desired slice plane is displayed on a display device. In order to obtain a plurality of tomographic images for a plurality of different slice planes of a patient by using the X-ray CT apparatus, the X-ray tube is rotated by 180 ° or 360 ° for the first slice plane and the first slice plane is rotated. After collecting all the projection data for the slice plane of the X-ray tube, the operation of the X-ray tube is stopped and the X-ray is placed in the vertical plane having the second slice plane.
Time must be spent horizontally moving the patient so that the tube is positioned, and then rotating the X-ray tube and X-ray exposure for the second slice plane. [0004] Therefore, in the case of obtaining a plurality of tomographic images for different slice planes, the conventional X-ray CT apparatus requires a long restraint time for the patient, and therefore the X-ray CT apparatus has There is a problem that operation efficiency is deteriorated. Further, in the conventional X-ray CT apparatus, when obtaining a plurality of tomographic images for different slice planes of a patient into which a contrast medium has been injected, the projection data is collected for the first slice plane and the projection data for the last slice plane. Since the physiological state of the patient changes between the time of acquisition and the time of acquisition, there is also a problem that a plurality of tomographic images cannot be obtained under the same physiological state. The present invention has been made in view of the above circumstances, and reduces the time required for feeding a subject during data collection, thereby reducing the time for binding a subject, and shortening the collection time for a plurality of slice planes. It is an object of the present invention to provide an X-ray CT apparatus capable of realizing an image. In order to achieve the above object, the present invention provides an X-ray CT apparatus for obtaining a tomographic image having a predetermined slice thickness on a desired slice plane of a subject. An X-ray source that emits X-rays that pass through the subject in a direction parallel to the rotation plane while rotating , an X-ray detector that detects X-rays that pass through the subject, and the X-ray source. Object moving means for moving the object in the body axis direction during the rotational movement of the X-ray source so as to advance by a distance of １ ／ of the predetermined slice thickness during one rotation; Data collecting means for collecting the obtained data, and image reconstructing means for reconstructing an image based on the data supplied from the data collecting means. The scanning can be performed so as to move the slice equivalent to the thickness of one slice tomographic plane in a short time. The present invention will be specifically described below with reference to examples. FIG. 1 is a system block diagram of an X-ray CT apparatus according to an embodiment of the present invention. 1
Is a gantry having an insertion hole 6 for inserting a subject M placed on the couch top 2 and an X-ray tube 3 as an X-ray source with the inserted subject M interposed therebetween. The line detector 4 is disposed to face. Here, the X-ray tube 3 is configured to control the generation of X-rays by the high-pressure generator 7 and to rotate around the insertion hole 6 by the X-ray tube drive control device 5. The X-ray detector 4 is configured by arranging a plurality of single detectors in an array along the circumferential surface of a cylindrical holding member disposed obliquely. Is always received by a part of the detector 4. Further, the couch top 2 can be continuously moved along the body axis direction of the subject M by the couch drive control device 8 (sometimes referred to as subject moving means). Reference numeral 9 denotes a data collection device for collecting data obtained by the X-ray detector 4, reference numeral 10 denotes a correction operation device for converting data in the data collection device into appropriate reconstructed data, and reference numeral 11 denotes a correction operation device. Reference numeral 12 denotes an image reconstructing apparatus for reconstructing an image based on data transmitted from the image reconstructing apparatus 10;
1 is a display device that performs display based on the image data from No. 1. Reference numeral 13 denotes a system control device that controls the above-described devices. Next, the operation will be described. In the above-mentioned apparatus, it is assumed that a fan beam-shaped X-ray (hereinafter, also simply referred to as a fan beam) is generated from the X-ray tube 3, and the X-ray detector 4 detects the fan beam as one unit. Further, it is assumed that the fan beam can be continuously rotated 10 times or infinitely continuously in this embodiment without stopping at 360 ° rotation. Such continuous rotation can be realized by using a known slip ring or by employing an electron beam scan as disclosed in US Pat. No. 4,158,142. The subject M moves continuously by the couch drive control device 8 while the fan beam continuously rotates and data is collected. This movement amount is, for example, advanced by Pmm per rotation of the fan beam. With this configuration, for example, it is equivalent to the fan beam rotating and translating in the body axis direction with respect to the stationary subject M, and the fan beam spirally moves around the subject M. Collect data (
(Spiral scan). That is, a spiral scan is performed by a combination of the movement of the X-ray source and the subject. The data obtained in this way can be defined as spiral data. Therefore, not only the CT device (4th generation CT device) that rotates only the X-ray tube 3 with the X-ray detector 4 fixed as in this embodiment, but also the X-ray tube and the X-ray The spiral data as described above can also be obtained by a CT device (third generation CT device) that relatively rotates and drives the device. When observing the positions of the X-ray source and the fan beam in the spiral scan from the direction perpendicular to the body axis and the direction perpendicular to the body axis, a sine waveform XL having a period Pmm as shown in FIG. 2 is drawn. Next, a method of reconstructing an image from the data obtained as described above will be described. First Typically, (1) method for reconstructing a time the total volume of the scanning range is divided into small elements, (2) eg from the slice point S ₁ in FIG. 2 in one revolution of the X-ray tube leading to S ₂ When considering the obtained data, the fan beam position is determined by the positions X ₁ and X _{2 in} the X direction in FIG.
A known method (US Pat. No. 4,149,247) of reconstructing an image for each plane by approximating the one fixed at the center of the image. Further, re data obtained by the two rotations leading to S ₃ from the point S ₁ as bundling by: Number 1 (overlapping with) one rotation of the data and scan representative of slice position X ₂ once by the data It can also be configured. ## EQU1 ## Here, θ is a rotation angle of the X-ray tube 3 and the X-ray fan beam FB as shown in FIG. 3, and takes a value of 0 to 360 °. P ₁₂ (θ, ψ) is the projection data relative position of the X-ray tube 3 to the subject M is obtained while reaching the S ₂ from S ₁ in FIG. 2, P ₂₃ (
theta, [psi) is also an X-ray tube relative position is the projection data obtained while leading to S ₃ from S _2. The above method can be deduced to a case where an image for one slice is obtained by three rotations or more. Further, the same effect as the above case can be obtained by independently reconstructing the projection data obtained in each rotation and averaging the obtained plural images. As described above, bundling data for a plurality of continuous rotations to form one image is useful in reducing artifacts. That is, in general, in an X-ray CT apparatus, the thickness of the X-ray fan beam viewed from the side does not become parallel X-rays.
The thickness becomes almost proportional from the wire tube. When the subject is examined with an X-ray beam as described above, if the subject has a large change in the slice thickness direction, an inconsistent portion is included for each angle θ at which the projection data is taken, and an artifact called a clipping effect is often generated. Will be born. A similar phenomenon occurs in the case of the present invention, which will be described with reference to FIG. In FIG. 4, A is an intensity profile in the slice thickness direction of the X-ray fan beam obtained when the X-ray tube is at the position S ₁ (ie, X = X ₁ ) in FIG. The slice thickness at this time is defined as tmm.
As the X-ray tube rotates, the slice plane moves in the body axis direction of the subject. For example, at θ = 180 °, the X-ray tube position is not at the initial position X ₁ , and P / 2 from there.
At a position advanced by (X = X ₁ + P / 2). Here, if P = t, the intensity profile and position of the X-ray fan beam in the slice thickness direction at θ = 180 ° are as shown in FIG. 4B. Here, the hatched portions in the waveforms A and B indicate that a non-common subject is measured. Since the image reconstruction calculation is performed on the assumption that all the projection data are the results of measuring exactly the same object, the hatched portions in the waveforms of A and B are considered to cause some distortion in the image. . This is true not only for the relationship between θ = 0 ° and 180 ° but also for the entire range of θ. In particular, in the data collection method as in this embodiment, many developments similar to the clipping effect are likely to occur. The present invention solves such a problem using the following principle. For example, tmm
In order to obtain the effective slice thickness, the subject M is sent at a rate of t / 2 mm per rotation of the X-ray fan beam, and the X-ray fan beam FB is narrowed down to t / 2 mm by a collimator or the like. . As a result, an intensity profile and a position as shown in FIG. 5 are obtained. In the figure, A and B are the intensity profiles and positions in the slice thickness direction of the X-ray fan beam obtained when the X-ray tube relative positions are X = X ₁ and X = X ₂ , respectively, and C and D are similarly numbers. 2 [0019] As a result, if the projection data is bundled as in the above equation 1, the profile and position as shown in FIG. 6 can be obtained. That is, E in the slice thickness direction of the projection data obtained at θ = 0 ° and 180 °, respectively.
And F are obtained. The hatched portion is relatively smaller than in the case of FIG. That is, image distortion is reduced. Further, when performing the spiral scan as described above, there are the following problems. θ
= 0 °, the collection of the projection data is started, and when the collection of the projection data for one image is completed at the position θmax substantially close to θ = 360 °, P (0, ψ
) And P (θmax, ψ) deviate from each other in the scan plane to be measured, so that the data contents are considerably different. It is well known that if there is a discontinuous difference between adjacent data, artifacts are more likely to occur than in the case of continuous deviation. In order to solve such a problem, the present invention includes a correction operation device 10 that performs the following processing. The principle of this correction arithmetic device is that one part obtained at the beginning or one part obtained at the end of the data to be used for image reconstruction of one tomographic plane (slice plane) is obtained before or after one part obtained at the end. The correction is performed using data obtained at the same rotation angle in the data of the tomographic plane. The projection data obtained from θ = 0 to θX is substituted by data P ′ (θ, ψ) that has been subjected to arithmetic processing as shown in the following equation (θX is the required image reconstruction area) It is arbitrarily set according to the size and the degree of reduction of artifacts). (Equation 3) Here, W (θ) is 0 at θ = 0 ° as shown in FIG. 7, and 1 at θ = θX,
The function is a function that connects between them by a straight line without a sharp change. Conversely, instead of such correction, the data P obtained by performing the arithmetic processing of the following equation 4 to correct the data obtained at θ = θY to θmax with the data obtained by the previous rotation
'(Θ, ψ) (θY has the same meaning as θX). (Equation 4) Here, W (θ) is a function in which θ = θY is 1 and θmax is 0, and the interval between them is, for example, a straight line without a sharp change. By providing such a correction operation device 10, adjacent data can be evaluated as a continuous deviation, so that the occurrence of artifacts can be reduced. The above correction some one image by extension for but which was in, then therewith two rotation or three rotating when creating an image that slightly extend therefrom and one rotation, from S ₁ to S ₂ Needless to say, it can be created. In the embodiment described above, if the relationship between the moving amount P of the bed per one rotation of the fan beam and the slice thickness t is selected as P <t, the deviation of the scan plane per rotation of the spiral is small. Therefore, the occurrence of artifacts is reduced. The present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above-described embodiment, the X-ray CT for forming one image from the projection data obtained from 0 to 360 ° has been described, but the X-ray CT apparatus as shown in FIG. 8 for performing image reconstruction from scan data of less than 360 ° Also applicable to That is, the X-ray source reciprocates or moves one way at a high speed on the orbit XL, and the detector group 4 'is arranged along a range of about 2/3 of the circumference. May be sent continuously.
In this case as well, it is possible to obtain projection data for one image from the X-ray source 3 'from a to b. According to such an apparatus, as shown in FIG. 9, a trajectory in which U-shaped scans are continuous can be obtained. The data obtained in this way can be defined as modified spiral data. In this case, in FIG.
'Must be moved so fast that the speed of movement from b to a (return) is negligible compared to the speed of movement from position a to b (during data collection). This can be sufficiently achieved by adopting the electron beam scanning. According to this embodiment, since the moving time of the X-ray source 3 'can be shortened, the slice interval Pmm can be minimized. Creating images for slices makes it easier to reduce artifacts. According to the present invention described in detail above, it is possible to reduce the time required for feeding the subject at the time of data collection to reduce the time for binding the subject, and to reduce the time required for the multiple slice plane. It is possible to provide an X-ray CT apparatus capable of shortening the acquisition time and reducing artifacts based on the clipping effect.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a system block diagram showing an embodiment of the present invention; FIG. 2 is a schematic explanatory diagram showing a relative trajectory of an X-ray source according to the embodiment; FIG. FIG. 4 is a schematic explanatory diagram showing a state of a beam. FIG. 4 is an explanatory diagram of a reason why distortion occurs in an image. FIG. 5 is an explanation of a reason why distortion occurring in an image can be reduced by employing the apparatus according to the embodiment of the present invention. FIG. 6 is an explanatory diagram of the reason why distortion generated in an image can be reduced by employing the apparatus according to the embodiment of the present invention. FIG. 7 is an explanatory diagram of a function used in a correction operation. FIG. 9 is a schematic explanatory view showing another embodiment. FIG. 9 is an explanatory view of the relative trajectory of the X-ray source according to the other embodiment. Explanation of reference numerals 1 gantry 2 couchtop, 3, 3 ′ X-ray source 4 X-ray detection Device 5 X-ray drive control device 6 Detector drive device 7 High voltage generator 8 Sleeper drive Controller 9 data collecting device 10 compensation calculation unit 11 image reconstructor 12 display device 13 system controller

Claims (1)

Claims: 1. An X-ray CT apparatus for obtaining a tomographic image of a predetermined slice thickness on a desired slice plane of an object, wherein the X-ray CT apparatus rotates around the object while rotating around the object.
An X-ray source that emits X-rays that pass through the subject in a row direction, an X-ray detector that detects X-rays that pass through the subject, and the predetermined slice during a time when the X-ray source makes one revolution. Subject moving means for moving the subject in the body axis direction during rotation of the X-ray source so as to advance by a distance of １ ／ of the thickness , and data collecting means for collecting data obtained from the detector And an image reconstructing means for reconstructing an image based on data supplied from the data collecting means.